This invention relates to electrical data pulse processing and, more particularly, to a circuit arrangement for improving the shape of a data pulse waveform which has suffered distortion during transmission.
It is well-known that when transmitting pulses which are nominally "square-wave", the available bandwidth of the transmission medium can so attenuate the high frequency components of such pulses that their pulse waveform, as received, tends to become rounded with sloping flanks. In other words, the original pulses to be transmitted contain high frequency components which are outside the available bandwidth of the transmission medium.
It is also well-known that both the leading and trailing flank slopes of a received pulse waveform can be increased, or steepened, by a technique known as "crispening" in which, in its basic form, an auxiliary waveform, obtained by a differentiation of a pulse waveform is added to the received pulse waveform to produce a resultant received pulse waveform having steeper flank slopes than the original received pulse waveform, albeit at the expense of unwanted overshoots which are introduced into the flank slopes.
United Kingdom patent specification No. 706 341 describes arrangements for "crispening" the instant pulse waveform of a television video signal. In a particular one of the arrangements which is described in this prior patent specification, the auxiliary waveform obtained by a differentiation of the instant pulse waveform is not added directly to that waveform, but instead the auxiliary waveform is clipped by a peak separator to produce upper and lower portions of the auxiliary waveform, and it is only these upper and lower portions which are added to the instant pulse waveform. The effect of adding only these upper and lower portions is to reduce the overshoots which are introduced into the flank slopes of the instant waveform, compared with the overshoots which would have been introduced had the entire auxiliary waveform been added to the instant pulse waveform. The clipping level for producing the upper and lower portions of the auxiliary waveform can be adjusted so as to vary the absolute amplitude of these portions.
In the publication Radio Mentor, March 1971, at pages 142-143, there is described a further arrangement which is for "crispening" a television video signal waveform and which can be considered as a refinement of the particular "crispening" arrangement discussed above. In this further arrangement, an auxiliary waveform obtained by a differentiation of the instant pulse waveform is clipped and then a second differentiation is performed on the clipped auxiliary waveform to obtain a resultant auxiliary waveform which is added to the instant pulse waveform to produce a resultant video signal waveform. The clippping level at which the auxiliary waveform is clipped is dynamically controlled. This dynamic control aims at reducing the overshoots introduced into the flank slopes of the resultant video signal waveform for a given increase in steepness.